|Home | About | Journals | Submit | Contact Us | Français|
The genetic structure of β-lactamases in Acinetobacter genospecies 3 (AG3) isolates in Taiwan was studied to analyze their high rates of resistance to β-lactams, including carbapenems (57.9%). blaIMP-1 and blaIMP-8 were located in a class 1 integron. blaOXA-58 was bracketed by ISAba3. A novel TnpF-like integrase gene was identified upstream of blaVEB-3. Adjacent to the 5′ sequence of the blaADC gene, folE was identified. Four new Acinetobacter-derived cephalosporinase (ADC) enzymes were found, which clustered phylogenetically with published AG3 ADC proteins.
Acinetobacter genospecies 3 (AG3), belonging to the A. calcoaceticus-A. baumannii complex, exists in the natural environment and the clinical setting. It was reported to be the most frequently isolated Acinetobacter species in Hong Kong, Sweden, and Ireland (3, 4, 32). Carbapenem-resistant or multidrug-resistant (MDR) AG3 has been found globally in the past decade (3, 12, 18, 20, 36).
In Acinetobacter spp., β-lactamases are the most common contributors to resistance to extended-spectrum cephalosporins and carbapenems, including extended-spectrum β-lactamases (ESBL), metallo-β-lactamases (MBL), carbapenem-hydrolyzing class D β-lactamases (CHDL), and Acinetobacter-derived cephalosporinases (ADC) (24, 27). Mobile genetic elements, including insertion sequences (IS) and integrons, may contribute to β-lactamase overproduction and dissemination (26, 31). Several carbapenemases (IMP-1, IMP-4, VIM-2, OXA-23, and OXA-58) and versatile AmpC cephalosporinases (ADC-12 to ADC-23) have been found in AG3 (2, 3, 12, 20, 22, 36) but with few studies on their genetic support to elucidate possible transmission mechanisms.
Nineteen AG3 clinical isolates were collected from three hospitals in Taiwan from 1999 to 2007 and were identified using the API 32GN kit (bioMerieux, Marcy-l'Etoile, France) and recA and rpoB sequencing (15, 16). Sixteen distinct pulsotypes were identified among the isolates, and each blood isolate displayed a different pulsotype, indicating nonclonal relatedness.
The MICs of antimicrobials were determined using the broth microdilution method, except the MIC of sulbactam, which was determined with agar dilution method (5). More than half of AG3 isolates were resistant to the β-lactams tested (57.9 to 94.7%), including carbapenems (57.9%), and all were susceptible to sulbactam, tigecycline, and colistin (Table (Table1).1). Fifteen isolates (78.9%) were resistance to ≥3 classes of antibiotics that were potentially effective against the Acinetobacter spp. and were classified as MDR AG3 (9). In this study, sulbactam, tigecycline, and colistin reveal their roles as effective agents for AG3, which is similar to the situation for carbapenem-resistant A. baumannii (19).
The β-lactamases and resistance determinants were detected by PCR (8, 13, 14, 20, 28, 30, 31, 34, 35), including Ambler class A ESBL genes (blaTEM, blaSHV, blaCTX-M, blaVEB, and blaPER), Amber class B MBL genes (blaIMP, blaVIM, blaGIM, blaSPM, and blaSIM), an Amber class C AmpC β-lactamase genes (blaADC), Amber class D carbapenemase genes (blaOXA-23-like, blaOXA24-like, blaOXA-51-like, and blaOXA-58-like), integrase genes (intI1, intI2, and intI3), IS elements (ISAba1, ISAba2, ISAba3, and IS18), transposase genes, and a putative replicase gene, repAci1. IS1008 was studied using the primers ATGGCATCAGCTATCGTG and CCTGTGCTCAATATCTACTGGA, and the putative TnpF-like integrase gene, identified in this study, was investigated with the primers GCAGACAGCTCTATCGTTAAAG and TTTGTATCACCTTCCTGCTG. The PCR amplification results showed that one isolate contained blaVEB (isolate 3), 11 harbored both blaIMP (10 had IMP-1 and 1 had IMP-8) and blaOXA-58-like, and all had blaADC. Twelve isolates (63.2%) harbored the class 1 integron but none had the class 2 or 3 integron. Eleven isolates (57.9%) had insertion sequence ISAba1, 3 (15.8%) had ISAba2, 18 (94.7%) had ISAba3, 2 (10.5%), had IS18, and 14 (73.7%) had IS1008. repAci1 was detected in one blaOXA-58-like-positive isolate and one blaOXA-58-like-negative isolate. The TnpF-like integrase gene was identified only in the blaVEB-positive isolate. The concomitant presence of MBL and CHDL was detected in 11 AG3 isolates (Table (Table2)2) that had been found in 1 A. baumannii strain in Taiwan and 2 in Greece (13, 33). Having both MBL and CHDL may not be rare in Acinetobacter, and its clinical impact is yet to be studied.
Sequence analysis of integron PCR products revealed that 11 isolates had blaIMP-aac6-II-aadA4 and/or dfrA1-orfC cassettes (Table (Table2),2), and 1 carried the aphA15-aadA1 cassette (isolate 3). The blaADC genes were PCR amplified with the primers FolE-F (CGTGCGGCTAAAGCTTT) and ADC-AG3-R (TTATTTCTTTATTGCATTYARYAC), designed from the A. baumannii strain AB0057 folE (1) and AG3 blaADC sequences (2). Four novel ADC enzymes (ADC-41 to -44) were identified and clustered (17) with 13 other AG3 ADC proteins (except ADC-5). Thirteen isolates (68.4%) had ADC-18, two had ADC-43, and the other four each had ADC-22, ADC-41, ADC-42, or ADC-44.
The genetic environment of the β-lactamase genes was sought by PCR mapping and plasmid random cloning (Fig. (Fig.1).1). The blaIMP-1 and blaIMP-8 genes are located in a class 1 integron and followed by the aminoglycoside-resistant genes aac6-II and aadA4 (Fig. (Fig.1A),1A), which appears to be a common genetic structure in Acinetobacter species (13, 36). Comobilization of the two classes of antimicrobial resistance genes by the same integron suggests the possibility of selecting carbapenem resistance by aminoglycoside use. The blaOXA-58 gene in AG3 is bracketed by ISAba3 (Fig. (Fig.1B),1B), which is structurally similar to previous reports for several different Acinetobacter genospecies (22) and A. baumannii on the plasmid pABIR (37). The prokaryotic type 1 GTP cyclohydrolase folE gene, associated with sulfonamide resistance in bacteria (7), was identified in the 5′-adjacent sequence of the ADC gene (Fig. (Fig.1C1C).
In A. baumannii, blaVEB-1 has been identified in cassette arrays of a class 1 integron with 3′-end conserved-segment (3′-CS) duplications in the French AYE strain (10), and blaVEB-1a is linked to ISCR2 in Argentinian strains (29). In a difference from the chromosome location reported previously (38), the plasmid location of blaVEB-3 in our isolate was identified using the I-CeuI mapping technique (28) and Southern hybridization of the plasmid. The upstream region of blaVEB-3 was cloned (21) and selected with 5 μg/ml ceftazidime (Sigma, St. Louis, MO). blaVEB-3 is located in a complex class 1 integron of 3′-CS duplications containing insertion sequence IS6100, the ISCR1 IS common region elements, and a novel integrase TnpF-like protein (Fig. (Fig.1D).1D). Furthermore, the TnpF-like integrase gene reveals as low a % G+C content (34.3%) as Acinetobacter (11), and its encoded protein exhibits high amino acid sequence identity to the TnpF protein from aquatic and soil bacteria. The TnpF-like integrase carried by the AG3 plasmid may provide a route for horizontal gene transfer between clinical and environmental strains.
Our results suggest that AG3 may obtain the antibiotic resistance genes, especially β-lactamase genes, from clinical or environmental Acinetobacter spp. through different gene acquisition mechanisms that include integrons, IS elements, and a novel integrase.
The nucleotide sequences of our reported ADC-type enzymes were deposited in GenBank under the accession numbers FJ744160 (ADC-41), FJ744161 (ADC-42), FJ744162 (ADC-43), and FJ744163 (ADC-44). The genetic surrounding sequences of β-lactamase genes identified were assigned the accession numbers GQ864268 (blaIMP), FJ744165 (blaOXA-58), FJ744164 (blaADC), and GQ926879 (blaVEB-3).
We are grateful to the microbiology laboratories of Taichung Veterans General Hospital, Kaohsiung Medical University Hospital, and Kaohsiung Municipal Hsiao-Kang Hospital for providing the isolates used in this study.
The National Health Research Institute and National Science Council (NSC 98-2314-B-037-042-MY3) supported this project.
No author has a conflict of interest to declare regarding the funding agencies and commercial companies. The funding agencies had no role in the design and conduct of the study, data analysis and interpretation, and preparation, review, and approval of the manuscript.
Published ahead of print on 5 April 2010.